The Use of Preheated Low-Enriched Air in Downdraft Gasifiers: Energy and Exergy Analysis

Abstract

In comparison with air, the use of oxygen as a gasifying agent is less exergetically efficient. For small-scale cogeneration plants, smaller than 1MW of thermal power, the use of oxygen is expensive and inefficient. The improvement in the technology of membrane separation of gases gives rise to the possible inclusion of oxygen-enriched air as a gasifying medium. The article aims at presenting a numerical study analyzing the gasification of treated wood in downdraft gasifiers with preheated lowenriched air (from 21 vol% to 30 vol%). The analyzed system consists of three subsystems: (i) for air enrichment by polymeric membrane, (ii) for preheating of enriched air by heat exchange with the product gas, and (iii) a downdraft gasifier. The increase of oxygen level in enriched air increases the temperature at the carbon boundary point (optimal gasification point), the amounts of combustible gases in the product gas, energy and exergy efficiencies and decreases the amount of air (oxygen) required for complete gasification as well as the amount of N2 in the product gas. The preheating of enriched air by heat exchange with the product gas is more beneficial for the gasification with lower levels of enrichment.